A cast having a hollow structure is used in various fields. For example, cooling holes provided to a turbine blade of a gas turbine are known to have a hollow structure that is especially complex in shape and is required to have high dimensional accuracy. A certain turbine blade includes cooling holes in an area covering from a dovetail portion (a joint portion) to a blade portion. Also, a big turbine blade further includes a shank portion (a body portion). Cooling holes are provided from the dovetail portion to the blade portion through the shank portion. The lost wax precision casting or the like is applied for such a cast having such a hollow structure. Such a big turbine blade can be manufactured by using, as a core, a ceramic sintered body having a shape corresponding to the hollow structure.
FIG. 1 illustrates a ceramic sintered body (a ceramic core 1) having a shape corresponding to cooling holes provided to an area covering from a dovetail portion (a joint portion) to a blade portion of a turbine blade, as an example of a ceramic sintered body used for producing a cast having a hollow structure. This ceramic core 1 roughly includes a blade portion 2 and a dovetail portion 3 both jointed through a platform portion 4. The blade portion 2 has a pin fin portion 5 that is particularly thin-walled. Other than this pin fin portion 5, there are many portions having a shape containing a curved surface or a curve and a tapered thickness. It is noted that when the cast having a hollow structure is a turbine blade having a shank portion, a ceramic core having a shape corresponding to the shank portion can be used.
There is disclosed a technology in which a ceramic sintered body having a shape corresponding to a complex hollow structure including such cooling holes of a turbine blade is formed by connecting and integrating a plurality of pieces. For example, Japanese Patent No. 3802095 (Patent Literature 1) discloses a ceramic sintered body used when a cast is a hollow blade including a rear edge portion and a body portion of a gas-cooled gas engine turbine. A first core portion corresponding to the rear edge portion and a second core portion corresponding to the body portion of the blade of this ceramic sintered body are connected to each other and integrated. A tongue-like element is disposed to one of the first core portion and the second core portion, and a groove-like element is disposed to the other of the first core portion and the second core portion. These elements are mated with each other so that the core portions are connected (for example, FIG. 2 of Patent Literature 1). Further, it is disclosed that when the first core portion and the second core portion have substantially equal thermal properties, a heated thermoplastic binder can be used as an adhesive, instead of disposing the above-described mating elements, so that the core portions are connected to each other (for example, claim 15 of Patent Literature 1). The first core portion and the second core portion connected in this manner are sintered in a state where they are integrated, so that a ceramic sintered body is formed.
In addition, Patent Literature 1 (for example, claim 15) discloses a technology (hereinafter, referred to as an “insert molding process”) in which the integrated ceramic molded body is formed by: placing in an injection molding mold a first core portion having one mating element (for example, a tongue-like element) on a connecting surface; and injecting a ceramic material corresponding to a second core portion into the residual portion in the mold. According to such an insert molding process, the subsequently injection-molded second core portion can be connected to the first core portion at the location of the mating elements in a state where spaces are eliminated. Therefore, it is considered that in the ceramic molded body before sintering or the ceramic sintered body after sintering including the first core portion and the second core portion in an integrated manner, separation and deviation in relative position between the first core portion and the second core portion are unlikely to be caused.